Why do they happen?
Earth's position around the sun
This drawing shows all the seasons for the Northern and Southern Hemisphere.
1= Northern Hemisphere- Summer. Southern Hemisphere- Winter.
2= Northern Hemisphere- Fall. Southern Hemisphere- Spring
3=Northern Hemisphere- Winter. Southern Hemisphere - Summer
Earth's tilt is 23.5%
Solsitices and Equinoxes
There are also solstice and equinoxes that mark the start of each season. For the Northern Hemisphere the winter solstice occurs on December 21st and is the shortest day of the year, it is in position 3 in the drawing. The summer solstice occurs on June 21st and is the longest day of the year, it is in position 1. The equinoxes are days with equal daytime and nighttime. The autumnal equinox is on September 21st and is in position 2 in the drawing. The Vernal equinox is on March 21st and is in position 4.